金属微成形损伤断裂的尺度相关性机理和多尺度模拟方法研究

The insufficiency of experimental and theoretical researches on the micro/meso-scale damage and fracture behaviors of polycrystalline metals, seriously impedes the development and application of micro-forming technologies. This project aims to develop a multi-scale ductile fracture model for micro-forming operations by using both experiments and crystal plasticity finite element method (CPFEM) based numerical simulations. The coupling effects of the micro/meso-scale features, such as the feature size of micro-parts, grain size and orientation, and free surfaces, as well as the grain-scale heterogeneous plastic deformation which significantly affect the micro-scale damage behaviors of metals will be fully investigated. This project also aims to seek out the physical mechanism of the size dependence of the ductile fracture of metals at micro-scale; answer the evolution law and the competitive behavior of void-dominated damage and shear-dominated damage. A specific ductile fracture model which includes both the void-dominated damage and the shear-dominated damage is developed and implemented into a dislocation density based CPFEM framework. Based on the multiplicative decomposition of deformation gradient, a damage deformation gradient which considers both the volume change and shape change is introduced, and the strong coupling between the elasto-plastic deformation and damage is then described. The developed multi-scale modeling synthetically considers the influence of the grain-scale heterogeneities of material and deformation on the damage and fracture behaviors of materials, and elucidates the competitive behavior of two damage mechanisms. Consequently, this project contributes to reveal the mechanism of the ductile fracture and failure of metals in micro-scale deformation, to predict forming limit of metals, and to provide the scientific guiding for planning micro-forming operations.

对材料微细尺度损伤断裂行为的实验和理论研究的欠缺，严重制约了微成形工艺的发展与应用。针对微成形中金属材料损伤断裂行为表现出明显的尺度相关性，传统损伤断裂模型难以进行有效描述的问题。本项目综合使用实验手段和考虑实际微观组织的晶体塑性有限元模拟，探索零件特征尺寸、晶粒大小、取向和自由表面相关的尺度效应和晶粒尺度的组织与变形不均匀性对材料损伤断裂行为的作用机制；厘清材料损伤断裂尺度相关性的物理机理和影响因素；阐明微细塑性变形中韧窝型和剪切型两种损伤机制的演化规律与竞争博弈关系；建立两种损伤机制的演化模型，并集成至基于位错密度的晶体塑性有限元框架内，构建适用于微细塑性变形的损伤断裂模型和多尺度模拟方法。通过项目研究，形成一套行之有效的微成形损伤断裂分析和研究工具，为断裂失效预报、成形极限预测和微成形工艺设计提供理论依据和数值模拟方法。

本项目综合使用前沿的实验技术和考虑实际微观组织的晶体塑性有限元模拟，研究试样几何特征尺寸、晶粒大小、取向和自由表面相关的尺度效应和晶粒尺度变形不均匀性对材料损伤断裂行为的作用机制，构建适用于微细塑性变形的损伤断裂模型和多尺度模拟方法。.研究了考虑位错密度和变形孪晶的大变形晶体塑性本构理论及基于反分析法的参数拟合方法，建立了“微观组织表征—单晶体细观力学行为辨识—组织演化动力学模型—力学性能/组织/缺陷预报”的集成计算材料仿真平台。.针对微观组织不均匀性和变形不均匀性对金属箔材损伤断裂尺度相关性行为，采用蒙特卡洛法建立可对比的微观组织代表体元，利用全场晶体塑性模拟技术系统研究微观组织和几何特征对介观尺度的应力/应变分布分配、应变集中与剪切带、晶格转动与取向演化、滑移系开动等行为的影响，厘清金属薄板塑性变形过程中损伤断裂行为的尺度相关性，为建立金属箔材损伤断裂模型提供重要的参考依据。.针对金属塑性成形损伤缺陷演化，基于跨层级建模技术，提出了从宏观塑性变形边界条件到考虑实际微观组织的多尺度模拟方法；结合准原位实验观察，研究微观组织对材料微区变形不均匀性及空洞演化的影响，建立了考虑了晶粒尺寸的空洞长大与聚合模型。阐明了不同晶粒尺寸下空洞长大的差异及对材料损伤断裂方式和塑性变形能力的影响机制。.通过实验与晶体塑性模拟研究，提出了考虑晶粒尺寸影响的空洞演化模型，是继1969年Rice和Tracey提出均匀材料的空洞长大模型以来，首次提出将晶粒尺寸影响考虑在内的非均匀材料空洞损伤模型。该模型影响空洞长大的因素不仅包含了材料力学性能，也包含了晶粒尺寸和微观组织的统计信息。.上述实验、理论和数值模拟研究针对典型面心立方、体心立方和密排六方金属开展，理论成果和所开发的多尺度计算平台具有普适性，为金属微塑性成形的断裂失效预报、成形极限预测和工艺设计提供理论依据和数值模拟方法。

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